It can go forever. VHO lighting (when you have MH) is supplemental lighting to make the lighting more pleasing to your eye and to add a sunrise/sunset period.
You tank will be fine with MH alone. When running MH, VHO's are mostly to provide color, (blue). MH puts out plenty of light, but unless you have 14,000K - 20,000K they can be more yellow than most people like so they add VHO or T-5 or PC.
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